This apparatus is directed to an AGC for use in an acoustic logging tool. This type of tool features an acoustic pulse transmitter. Acoustic pulses are transmitted into the well fluid and pass into the adjacent formations. Acoustic signals are observed at receivers mounted on the logging tool. Typically, more than one acoustic receiver is used. One arrangement features a first spaced receiver and an identical receiver which is spaced even further away from the acoustic transmitter. This receiver system enables a comparison to be made between the received signals at the two receivers. Data reduction is simplifier by procedure which is known as acoustic amplitude ratio logging. This involves the use of signals from two acoustic receivers. The ratio of the signals is more important than the absolute valves in this two receiver system. Other data processing procedures involving the signal from a single receiver are enhanced by the AGC of the present disclosure.
The method of operation of an acoustic logging tool normally involves the propagation of an acoustic burst transmitted from a piezoelectric or magnetostrictive transducer. The transmitted pulse is typically very short. The signal at receivers in the acoustic logging tool has a wide dynamic range. The pulse return to the receiver is relatively dynamic in light of this fact. Typically, the dynamic range of the received signal is quite wide, easily having a range of 1,000 fold or greater.
While it is possible to utilize AGC circuits with dynamic ranges up to one million to one, it is equally difficult to have only a limited AGC gain range to accommodate a wide dynamic range. Such a limit is in part imposed by data processing apparatus. One such requirement arises in the display of the received data in a display form utilizing the system known as Micro-Seismogram, a service mark identifying such a display system. A typical range of data presentation is about 16 gray levels. This data handling format has achieved a high degree of popularity.
To accommodate the more limited dynamic range exemplified by 16 levels, or any other limited dynamic range, the AGC system must provide a relatively limited response. It is desirable that the AGC have a limited dynamic range. In addition to that, this apparatus limits the dynamic range only for a specified interval or window of time. The window is tied to the event being observed. In this instance, it is preferably to determine the window based on the incoming signal at the receiver. With this in mind, a window is thus defined based on increase in the signal at the receiver, thereby outputting data in a compressed data range where the time window limits the operation of the circuit to a time when data is more relevant. Extraneous or system noise occurs between signals and is blanked off by the time window determined by the circuit, thereby limiting operation of the equipment only to those intervals at which time signal data will be processed.
The present apparatus is summarized as an AGC control circuit including an input operational amplifier with a suitable full wave rectifier. The rectified positive and negative peaks are amplified and applied to the output of the circuit. Peaks are then detected by a peak detector, including both positive and negative peaks. This output is supplied through an amplifier then to a sample and hold amplifier. The sample and hold amplifier forms a feedback control signal applied to the input amplifier, altering its gain and hence the gain of the closed feedback loop.